1. Field of the Invention
This invention relates to non-bleeding, predispersed liquid or low melting solid peroxide vulcanizing agent or coagent compositions and to processes for their preparation.
2. Description of the Prior Art
During the manufacture of rubber or plastic articles, numerous rubber or plastic compounding chemicals, such as peroxide vulcanizing agents and coagents, accelerators, activators, vulcanizing agents, antidegradents, stabilizers, and the like, must be added to the rubber or plastic stock. These compounding chemicals are generally in the form of finely divided dry powders, liquids and low melting solids. They are conventionally incorporated into the stock using internal mixers, such as Banbury mixers, rubber mills or extruders. In the use of such masticating equipment, the chemicals are blended directly with the rubber or plastic stock or masterbatches of these chemicals are added during mixing.
Most of these compounding chemicals are in the form of finely divided dry powders which can become a health hazard to all persons who must work with them from the dry grinding operation to the compounding stage. During all of the mechanical manipulations of these powdered chemicals, such as packaging, transportation, unpacking, weighing and adding to the mixing vessel, the issuing dust clouds represent wasted chemical which leads to inaccurate addition adversely affecting the reproducibility of the cure rate and physical properties of the product from batch to batch. Most of these compounding chemicals are highly reactive, undergo hydrolysis and oxidation reactions, and some are undesirably deliquescent. In addition to the chemical lost as a dust cloud or vapor during mixing, some may fall through the rolls during mill mixing.
These problems associated with handling, mixing and adequately dispersing powdered chemicals into tough polymer matrices have plagued the rubber and plastic industries since their inception. Two particularly suitable techniques for preparing predispersions containing very high concentrations of such compounding chemicals to eliminate these problems are disclosed in U.S. Pat. Nos. 4,092,285 and 4,110,240.
An equally troublesome problem to these industries has been the handling, mixing and dispersing of liquid and low melting solid compounding chemicals. A low melting solid compounding chemical is one that melts at or below handling or processing temperatures, generally considered in the rubber or plastics industries to be below about 140.degree. F. (60.degree. C.). Many such compounding chemicals are employed as peroxide vulcanizing agents and coagents, and polymerization catalysts for rubber and plastic compounds. They are all highly reactive and many are strong oxidizing agents. During processing, when compounding chemicals that are liquids or that melt at or below mill temperatures, such as organic peroxides, are added to stock on the mill, the band of stock on the roll breaks and the mixture falls off of the roll into the pan. The stock is then in the form of a slippery mass that is difficult to band again on the mill. When these liquid or low melting solid compounding chemicals are added to internal mixers, they have a tendency to lubricate the stock causing slippage and a reduction in shear which leads to longer mixing times. They also are sprayed around the inside of the mixer and cake up on the surfaces of the equipment.
Compounding chemicals in liquid or molten form, particularly those which are very viscous, are difficult to transfer from one container to another with any degree of accuracy because of incomplete transfer. Viscous liquids containing more than one ingredient also have a tendency to striate, which leads to concentration gradients upon sitting and to inaccurate additions of the active material. In addition, residual chemicals on packages and weighing containers tend to pick up contaminants, adhere to the handler's body and clothing, and in general, produce unpleasant and often hazardous working conditions.
One such low melting solid organic peroxide which has been found to be particularly difficult to handle is dicumyl peroxide. Dicumyl peroxide is used primarily as a curing agent or vulcanizing agent for many rubber or plastic polymers, such as polyethylene, ethylene-propylene copolymers (EPM), ethylene-propylene-diene terpolymers (EPDM), nitrile rubbers, polybutadiene rubbers, styrene-butadiene copolymer (SBR), chlorinated polyethylenes, chlorosulfonated polyethylenes, natural rubber, polyisoprene rubber, neoprene rubber, silicone rubber, acrylic rubber, polysulfide rubber, polyurethane, acrylonitrile-butadiene-styrene, and polyester. Most elastomeric polymers can be cross-linked with dicumyl peroxide with the exception of butyl rubber, polyisobutylene, epichlorohydrin rubber, polypropylene, polypropylene oxide rubber, and polyvinyl chloride in which chain scission occurs in the presence of a peroxide. Halogenated polymers must be fully stabilized from dehalogenation decomposition products before they can be cross-linked with dicumyl peroxide.
Dicumyl peroxide is a low melting solid which is available in two grades, a technical grade which is 90-96 percent pure and has a melting point of about 85.degree. F. and a recrystallized grade which is 96-100 percent pure dicumyl peroxide and has a melting point of 100.6.degree. F. Because it has such a low melting point, dicumyl peroxide in the form of granules melts readily in containers during the warm conditions often encountered in shipment and storage. In fact, the pressure encountered in a standard drum container is enough to quickly fuse the granules together. When the melted peroxide cools, it hardens into a block in the shape of the container. If the compounder wishes to use some of the peroxide, he must then break chunks of the peroxide from the block. The peroxide may also be melted and handled as a liquid. This is hazardous because the temperature must be carefully controlled to avoid decomposition or possible exothermic reaction. Moreover, the melted peroxide tends to splash and to refreeze easily and stick to the container. This often leads to inaccurate additions of the peroxide to the stock from batch to batch, resulting in poor reproducibility of the cure rate and the physical properties of the vulcanizates.
In order to obtain useful cured products when employing dicumyl peroxide, it is important that a uniform controlled amount of the peroxide be dispersed in the compound and that the compound be exposed to a uniform amount of processing heat history. When rubber is processed, it generates heat from the friction of the large macromolecules of rubber rubbing against one another. Normal temperatures encountered during mixing of rubber range from about 150.degree. F. to 400.degree. F. When dicumyl peroxide is added to the rubber compound being mixed, the warm rubber quickly melts the dicumyl peroxide. The melted peroxide lubricates and breaks up the batch of rubber compound, splashes out of the mixing unit, and becomes stuck to the cold metal parts of the mixing unit.
Similar problems are encountered in the handling and processing of another low melting solid organic peroxide, viz. .alpha.,.alpha.'-bis(t-butylperoxy)diisopropylbenzene. This peroxide, which is listed as having a melting point in the range of 122.degree.-176.degree. F., although melting points in the range of 113.degree.-131.degree. F. are typical, is likewise employed primarily as a vulcanizing agent or curing agent for the same rubber or plastic polymers that can be cured by dicumyl peroxide.
A number of attempts have been made to prepare products intended to eliminate the problems associated with the handling and processing of low melting solid peroxide compounding chemicals, such as dicumyl peroxide and .alpha.,.alpha.'-bis(t-butylperoxy)diisopropylbenzene. For example, dicumyl peroxide has been let down on a diluent such as Burgess KE clay and calcium carbonate at a concentration of about 40 percent active chemical. These products are commercially available as Di-Cup.RTM. 40KE and Di-Cup.RTM. 40C from Hercules, Inc. and Peroximon DC 40 from Montedison, S.p.A. .alpha.,.alpha.'-bis(t-butylperoxy)diisopropylbenzene is commercially available at a 40 percent active concentration let down on Burgess KE clay. Examples of such products include Vul-Cup.RTM. 40KE from Hercules, Inc., Peroximon F 40 from Montedison, S.p.A., Percadox 14/40 from Noury Chemical Co., and Trigonox 14/40 from Akzo Chemie Nederland.
Although these products behave as solids at temperatures above the melting point of the dicumyl peroxide and the .alpha.,.alpha.'-bis(t-butylperoxy)diisopropylbenzene, they suffer from a number of serious drawbacks. First, the activity of the product is relatively low so that a large amount of filler material is added to the stock. This filler material not only adds unwanted ingredients to the final product made from the stock but also increases the cost of compounding the stock since more of these products must be used to obtain the same amount of desired peroxide. If the polymer is generally transparent, such as polyethylene, the large amount of added filler material renders the final product hazy or translucent to opaque. Second, since the filler material is a rigid inorganic substance rather than an elastomeric or polymeric substance, it tends to detract from the desired physical properties of the final product. Third, these products tend to striate. During repeated melting and freezing of the peroxide during shipping and storage, the peroxide tends to separate from the filler which settles gradually toward the bottom of the container resulting in a greater concentration of peroxide at the top of the container. This difference in concentration of peroxide makes it difficult to obtain the same amount of peroxide from batch to batch. The portion with the higher concentration of filler tends to be powdery, and therefore, is sprayed around the internal mixer during the mixing operation. Fourth, since the peroxide is on the surface of the inorganic material, the product will bleed during shipping where temperatures above the peroxide melting point are encountered. When the temperature goes below the melting point, the particles tends to agglomerate.
Some of these drawbacks were overcome by mechanically dispersing the dicumyl peroxide in ethylene-propylene copolymer (EPM) and ethylene-propylene-diene terpolymer (EPDM). Such products are available commercially as POLY-DISPERSION.RTM. E(DIC)D-30, E(DIC)D-40, and T(DIC)D-40 from Wyrough and Loser, Inc. The concentration of dicumyl peroxide in the predispersion has been limited to no more than about 40 percent by weight, however, because of the basic incompatibility between the dicumyl peroxide and the polymer. This incompatibility is manifested by a "bleeding" of the dicumyl peroxide to the surface which imparts a greasy feel at temperatures above 100.degree. F. and a white crystal growth at temperatures under 100.degree. F. Care must be taken to avoid losing this "grease" or "cyrstal" since it is active dicumyl peroxide. Moreover, these polymeric dispersions of dicumyl peroxide still suffer from the disadvantage of having a relatively low peroxide activity.
Therefore, for these reasons, none of the prior art methods for improving the handling and processing characteristics of liquid or low melting solid peroxide vulcanizing agents and coagents has been completely satisfactory.